A variety of electronic circuits are used to drive loads and, more particularly, to control electrical current through strings of series connected light-emitting diodes (LEDs), which, in some embodiments, form an LED display, or, more particularly, a backlight for a display, for example, a liquid crystal display (LCD). It is known that individual LEDs have a variation in forward voltage drop from unit to unit. Therefore, the strings of series connected LEDs can have a variation in forward voltage drop.
Strings of series connected LEDs can be coupled to a common DC-DC converter, e.g., a switching regulator, e.g., a boost switching regulator, at one end of the LED strings, The switching regulator can be configured to provide a high enough voltage to supply each of the strings of LEDs. The other end of each of the strings of series connected LEDs can be coupled to a respective current sink, configured to sink a relatively constant current through each of the strings of series connected LEDs.
It will be appreciated that the voltage generated by the common switching regulator must be a high enough voltage to supply the one series connected string of LEDs having the greatest total voltage drop, plus an overhead voltage needed by the respective current sink. In other words, if four series connected strings of LEDs have voltage drops of 30V, 30V, 30V, and 31 volts, and each respective current sink requires at least one volt in order to operate, then the common boost switching regulator must supply at least 32 volts.
While it is possible to provide a fixed voltage switching regulator that can supply enough voltage for all possible series strings of LEDs, such a switching regulator would generate unnecessarily high power dissipation when driving strings of series connected LEDs having less voltage drop. Therefore, in some LED driver circuits, the voltage drops through each of the strings of series connected LEDs are sensed (for example, by a so-called “minimum select circuit,” or by a multi-input amplifier) to select a lowest voltage or lowest average voltage appearing at the end of one of the strings of series connected LEDs. The common switching regulator is controlled to generate an output voltage only high enough to drive the series connected led string having the lowest voltage (i.e., the highest voltage drop) or to drive a lowest average voltage to the strings. Arrangements are described, for example, in U.S. Pat. No. 6,822,403, issued Nov. 23, 2004, and in U.S. patent Ser. No. 12/267,645, filed Nov. 10, 2008, and entitled “Electronic Circuits for Driving Series Connected Light Emitting Diode Strings.”
It will be understood that a predetermined current can be regulated through each one of the series connected diode strings, and the voltage of the DC-DC converter can be maintained just high enough to drive a worst case one of the diode strings, or to drive a worst case average voltage through the diode strings.
In some applications, it is desirable to dim or to brighten the LED diode strings. In some particular applications, it is desirable to brighten and to dim the LED diode string over a wide dynamic range.
In order to cause a dimming or brightening of the LEDs while still maintaining a desirable lowest voltage from the DC-DC converter (switching regulator), and while still maintaining the predetermined current through the diode strings, the predetermined current through the LEDs can be cycled on and off at a rate fast enough to be undetected by the human eye. When the current through the LEDs is on, the current equals the desirable predetermined current, and when the current through the LEDs is off, the current can be zero or some current less than the predetermined current.
When the current through the load is switched off, it is desirable to switch off the DC-DC converter, and when the current through the load is switched on, it is desirable to switch on the DC-DC converter. If the DC-DC converter is left on when the current through the load is switched off, the DC-DC converter would lack feedback control and the output voltage of the DC-DC converter could move to a different voltage, which is undesirable.
Since, as described above, the common switching regulator is controlled to generate an output voltage only high enough to drive the series connected LED string having the lowest voltage, any voltage dip in the output voltage of the switching regulator can cause a corresponding dip in current supplied to one or more of the series connected LED strings, resulting in an undesirable flicker. The voltage dip can occur in the output voltage of the switching regulator due to a variety of causes. For example, a dip in the regulated output voltage of the switching regulator can occur due to a dip in the input voltage supplied to the switching regulator. In this case, the regulated output voltage would not recover for some period of time, until the switching regulator recovers, and the flicker would occur.
In order to achieve the wide dynamic range of brightness required by some applications, the on time of the current and the on time of the DC-DC converter must be able to be very short. For reasons described below, DC-DC converters are unable to achieve very short on times when switched on and off.
A DC-DC converter is often used in a feedback arrangement, in which a current or voltage at a load is sensed and the sensed current or voltage is used in a feedback loop to control the output voltage of the DC-DC converter. In a feedback loop, there is often so-called “compensation,” often in the form of a capacitor or filter, in order to slow the response time of the feedback loop in order to maintain stability.
Furthermore, many types of DC-DC converters, and switching regulators in particular, use an inductor to store energy during operation. The DC-DC converter, and the inductor in particular, require a finite time to reach steady state operation, and to reach a steady state output voltage.
In view of the above, it should be recognized that, when a short on time is desired to achieve a wide brightness dynamic range, the DC-DC converter may not behave properly in short duty cycle operation and fluctuations of the output voltage of the DC-DC converter may result. Thus, for a very short duty cycles, fluctuations in the output voltage of the DC-DC converter can also result in flicker.
It would be desirable to provide a circuit and technique that can achieve a wide dynamic range of power provided to a load by a DC-DC converter in a feedback loop arrangement, while reducing any fluctuation of power delivered to the load (e.g., flicker) that may result from fluctuation of a regulated voltage supplied by the DC-DC converter.